This invention has for its principal objective the provision of a material that has relatively good electrical and thermal conductivity, and, for example, a low coefficient of thermal expansion or a high hardness, or high wear resistance, magnetic properties, etc. Achievement of these objectives is accomplished by blending powders of (a) a preformed dispersion strengthened metal, e.g., dispersion strengthend copper, silver, or aluminum desirably having an electrical resistivity below 8.times.10.sup.-6 ohm-cm and (b) a different hard metal or hard metal alloy, e.g., one having a low coefficient of expansion, i.e., below 10.times.10.sup.6 /.degree.C. at 20.degree. C. or a metal alloy, e.g., iron-nickel alloys containing from 30% to 55% nickel by weight and minor additives such as manganese, silicon and carbon, etc., and compacting without a sintering step to substantially full density. By "preformed" as used herein is meant that the dispersion strengthened metal is provided as a dispersion strengthened metal powder before blending with component (b).
Dispersion strengthened metals are well known. Reference may be had to Nadkarni et al U.S. Pat. No. 3,779,714 and the references discussed in the text thereof for examples of dispersion strengthened metals, especially copper, and methods of making dispersion strengthened metals. The disclosure of U.S. Pat. No. 3,799,714 is incorporated herein by reference. In this patent, dispersion strengthened copper (hereinafter called "DSC") is produced by forming an alloy of copper as a matrix metal and aluminum as a refractory oxide forming solute metal. The alloy containing from 0.01% to 5% by weight of the solute metal, is comminuted by atomization, (See U.S. Pat. No. 4,170,466) or by conventional size reduction methods to a particle size, desirably less than about 300 microns, preferably from 5 to 100 microns, then mixed with an oxidant. The resultant alloy powder-oxidant mixture is then compacted prior to heat treatment, or heated to a temperature sufficient to decompose the oxidant to yield oxygen to internally oxidize the solute metal to the refractory metal oxide in situ and thereby provide a very fine and uniform dispersion of refractory oxide, e.g., alumina, throughout the matrix metal. Thereafter the preformed dispersion strengthened metal is collected as a powder or submitted to size reduction to yield a powder having a particle size of from -20 mesh to submicron size for use herein. Mechanical alloying of the matrix and solute metals as by prolonged ball milling of a powder mixture of 40 to 100 hours can also be used prior to internal oxidation.
Dispersion strengthening can be accomplished in a sealed can or container (U.S. Pat. No. 3,884,676). The alloy powder may be recrystallized prior to dispersion strengthening (U.S. Pat. Nos. 3,893,844 and 4,077,816). Other processes are disclosed in U.S. Pat. Nos. 4,274,873; 4,315,770 and 4,315,777. The disclosures of all of the foregoing U.S. Patents are incorporated herein by reference thereto. These patents are commonly owned with the present application.
Composites of metal powders having low thermal expansion characteristics and low resistivity are known.
Reference may be had to U.S. Pat. No. 4,158,719 to Frantz. According to this patent, a composite is made by compacting a mixture of two powders, one of which has low thermal expansivity and the other of which has high thermal conductivity. The composite is useful, as are the products of the present invention, in the production of lead frames for integrated circuit chips. Frantz's composite is made by mixing the powders, forming into a green compact, sintering and then rolling to size. The low thermal expansivity alloy is 45 to 70% iron, 20-55% nickel, up to 25% cobalt and up to 5% chromium. The high thermal conductivity metal is iron, copper, or nickel. None of the metals is dispersion strengthened. The nickel/iron alloy containing 36% Ni, balance Fe with Mn, Si and C totalling less than 1% is known as "Nilvar" or "Alloy 36". The nickel/iron alloy containing 42% nickel, balance Fe with Mn, Si and C totalling less than 1% is a member of a family of nickel/iron alloys known as Invar. It is also known as Alloy 42. The nickel/iron alloy containing 46% Nickel, balance Fe with Mn, Si and C totalling less than 1% is known as Alloy 46. Similarly Alloys 50 and 52 comprise 50% Ni and 52% Ni, respectively, balance Fe.
The respective properties of the sintered composites of the prior art and the unsintered composites of the present invention have been studied.
A composite strip and wire made with DSC and copper and each of (1) 36% Ni/64% Fe and (2) 42% Ni/58% Fe Invar type alloys, respectively. The powders were blended 50:50 and the respective procedures followed for forming the composites. Those composites made with DSC and the Invar alloys have high strength and good strength retention after exposure to high temperatures. The prior art material iron with alloy (1) and iron with alloy (2) shows higher strength than copper metal with alloys (1) or (2), but this is only with the sacrifice of electrical conductivity.
To obtain high strength with copper composites, the prior art has to use fine powder which reduces conductivity significantly. Coarse copper powder yields high conductivity but lower strength.
Another example of the prior art is the patent to Bergmann et al U.S. Pat. No. 4,366,065. This patent discloses the preparation of a composite material by powder metallurgy wherein a starting material comprised of at least one body-centered cubic metal contaminated by oxygen in its bulk and on its surface is mixed with a less noble supplemental component having a greater binding enthalpy for oxygen in powder form or as an alloy whereby the oxygen contaminant becomes bound to the supplemental component (aluminum) by internal solid state reduction. The composite is then deformed in at least one dimension to form ribbons or fibers thereof. Niobium-copper is exemplified with aluminum as the oxygen getter.
A principal advantage of using DSC as opposed to using plain copper appears to be that DSC enables closer matching of stresses required for deformation of the two major components. Because of this closer matching, the powder blends and composites can be co-extruded, hot forged, cold or hot rolled and cold or hot swaged. When one of the components undergoing such working is excessively harder, for example, than the other, then the particles of the harder component remain undeformed. The flow of softer material over and around the harder particles generally leads to the formation of voids and cracks, and hence weakness in the structure. The greater strength of the DSC material over the unmodified or plain copper enables closer matching with the hard metal as, for example, with respect to yield strength, and the size and shape of the regions occupied by the individual components will be more nearly alike. Closer matching of forming stresses enables achievement of full density for the powder blend in one hot forming operation, such as extrusion, or multiple size reduction steps such as swaging or rolling. This eliminates the need for sintering. The prior art utilizes two sintering steps at very high temperatures (1850.degree. F. for copper and 2300.degree. F. for iron). These temperatures promote inter-diffusion of atoms of the two components, or alloying, to occur. Diffusion of iron and/or nickel or other metals into copper lowers the electrical conductivity of the copper and conversely, diffusion of copper into the hard metal adversely effects its coefficient of thermal expansion.
In carrying out the present invention the temperatures encountered are below sintering temperature used in prior art procedures and inter-diffusion of atoms, or alloying, between the principal components is reduced. From the prior art it is evident that when sintering time is increased from 3 minutes to 60 minutes, the electrical resistivity does increase significantly from 35 up to 98 microhm-cm. (See examples 4 and 6 and examples 5 and 7 U.S. Pat. No. 4,158,719). Stated in another way, electrical conductivity decreases significantly. This variation in resistivity or conductivity indicates that inter-diffusion of copper and nickel (for example, from Invar alloy 42) is a serious problem. Use of DSC instead of copper or a copper alloy retards such inter-diffusion because the dispersed refractory oxide, e.g., Al.sub.2 O.sub.3 acts as a barrier to or inhibitor of diffusion. DSC (AL 15) has an electrical conductivity of 90-92% IACS and an annealed yield strength of 50,000 psi.
Other patent references of interest include Mackiw et al U.S. Pat. No. 2,853,401 which discloses chemically precipitating a metal onto the surface of fine particles of a carbide, boride, nitride or silicide of a refractory hard metal to form a composite powder and then compacting the powder. Hassler U.S. Pat. No. 4,032,301 dicloses a contact material for vacuum switches formed of mixed powders of a high electrical conductivity metal, e.g., copper, and a high melting point metal, e.g., chromium, compacted, and sintered. Bantowski, 4,139,378 is concerned with brass powder compacts improved by including a minor amount of cobalt. The compacts are sintered. Cadle et al U.S. Pat. No. 4,198,234 discloses mixing a pre-alloy powder of chromium, iron, silicon, boron, carbon and nickel at least about 60%, and copper powder, compacting the blend and sintering at 1050.degree. C. to 1100.degree. C. to partly dissolve the copper and nickel alloy in one another.
The present invention is distinguished from the prior art particularly in that it utilizes a preformed dispersion strengthened metal, e.g., DSC, dispersion strengthened aluminum or dispersion strengthened silver. The product of this invention in addition to having relatively high electrical conductivity, has improved mechanical properties not possessed by the prior art composites. The material is compacted to substantially full density without a sintering step.